By convention, it is considered that a bending test in pure bending is a bending test that is implemented while inducing as little parasitic force as possible into the bending zone of the testpiece, i.e. more specifically as little normal and/or intersecting force as possible.
In the context of the present application, when it is said that the turning movement is optionally alternating, or that a test in pure-bending is optionally in alternating bending, that constitutes a convenient shorthand for specifying in particular that:                the changes in turning during a test may be monotonic, i.e. always in the same direction, or with one or more reversals of direction; and        if there is a reversal of direction, the maximum amplitudes in opposite directions may be equal or different.        
Such a method is described in an application to performing alternating pure bending tests by M. Brunet, F. Morestin, and S. Godereaux (2001, “Non-linear kinematic hardening identification for anistropic sheet metals with bending-unbending tests”, Journal of Engineering Materials and Technology, Vol. 123, pp. 378–383), who also describe apparatus and a machine for implementing the method.
That known method is applied to a single testpiece, as do all previously-known methods of alternating bending testing. Nevertheless, in a manner specific to that method, each of the grip zones of the testpiece is held securely in a respective clamp mounted to pivot about a respective axis in a respective slider and engaged with a common device for driving both clamps and suitable for imparting thereto, and also to the grip zones of the testpiece to which they are respectively secured, alternating opposite turning movements about the respective pivot axes relative to the respective sliders so as to impart alternating bending to the bending zone of the testpiece between the clamps. The pivot axes of the two clamps are mutually parallel and the two sliders are mounted to slide on a common slideway in a direction perpendicular to the two pivot axes, thus allowing the axes to move towards each other or apart from each other along said direction as a function of variation in the apparent length of the testpiece between the two grip zones, i.e. between the two clamps, depending on the bending state of its bending zone.
The pivotal mounting of each clamp about the respective pivot axis in the respective slide takes place via a respective shaft, which shaft is secured to each clamp to lie on the corresponding pivot axis, and is engaged in two bearings of the corresponding slide. Between these two bearings, the shaft meshes via a respective gear train with a respective drive shaft, itself mounted to turn in two bearings of the corresponding slide about an axis that is parallel to the respective pivot axis and that is disposed relative thereto in such a manner that the axes of rotation of the drive shafts corresponding to the two slides, i.e. corresponding to the two clamps, are further apart from each other than are the pivot axes of the clamps. Each drive shafts itself acts via an Oldham joint disposed opposite from the corresponding clamp relative to the corresponding slide, to engage a respective outlet shaft of the drive device which is constituted by an electric motor associated with a torque limiter.
The bending zone of the testpiece can thus be subjected to bending alternately in one direction and in the other, through an amplitude that is adjusted by adjusting the magnitude of the turning of each clamp about the corresponding pivot axis relative to the corresponding slide, said magnitude of turning being identical at all times for both clamps because they are driven in common.
In that known apparatus, the resistance opposed by the clamps and by the grip zones of the testpiece against alternating turning is measured by sensors disposed on the drive shafts between the Oldham joints and the slides, in order to measure the twisting stresses of the drive shafts, where changes in such resistance serve to deduce changes in the resistance to bending of the bending zone.
That prior art apparatus makes it possible continuously to control the pivoting of each grip zone about its pivot axis, i.e. the bending of the bending zone between the grip zones, thereby constituting a significant advance over earlier apparatuses, in particular over the apparatus which appears previously to have been the most satisfactory in terms of maximum bending amplitude, in particular on a testpiece of small thickness, as measured perpendicularly to its mean surface, i.e. the apparatus described by F. Yoshida, M. Urabe, and V. V. Toropov (1998, “Identification of material parameters in constitutive model for sheet metals from cyclic bending test”, International Journal for Mechanical Sciences, Vol. 40, pp. 237–249).
The apparatus described by Yoshida et al. acts positively by means of a drive motor in alternating turning only on a first one of the grip zones of the testpiece, while the second grip zone is merely held at a determined orientation relative to a frame that also carries the motor by means of slideway-and-slider assemblies allowing it to move along two mutually perpendicular directions in order to allow the first grip zone to change its direction and in order to accommodate variations in the apparent length of the bending zone between the two grip zones while alternating bending is being applied.
In the apparatus of Yoshida et al., the bending zone thus serves as means for transmitting movement from the grip zone that is directly connected to the motor for drive in alternating turning motion to both the other grip zones, and also the slider-slideway assemblies that serve to maintain a constant orientation, and as a result the non-negligible friction that appears in the connections between the sliders and the slideways leads to non-negligible interfering forces appearing in the testpiece and more precisely in its bending zone, so bending conditions remain remote from ideal conditions of pure bending. This leads to a non-negligible amount of error in determining the changing resistance to bending of the bending zone from a measurement of the resistance opposed to the alternating turning movements by the grip zone that is connected to the motor.
Simultaneous positive action in pivoting on both grip zones of the testpiece enables the two grip zones to be guided in a single direction only, i.e. in practice it enables the two clamps to be guided to slide relative to each other in a single direction only in the apparatus of Brunet et al., thereby enabling friction to be reduced in comparison with the apparatus of Yoshida et al., and consequently reducing the parasitic forces induced in the bending zone by the friction and the disturbances that follow therefrom when studying changes in the bending resistance of the bending zone, with the study thus being less remote from pure bending, but with the friction and the parasitic forces still remaining perceptible. In other words, the twisting stresses measured on the drive shafts of the apparatus of Brunet et al. are due not only to the bending resistance of the bending zone, but also to the resistance due to friction that the slides encounter against the slideway whenever they need to move towards each other or apart from each other as a function of variations in the apparent length of the bending zone between the grip zones, i.e. between the clamps; in addition, the twisting stresses are also associated in part with the resistance opposed to turning by the shafts carrying the clamps in their bearings in the slides, by the gearing transmitting motion between these shafts and the drive shafts, and by the drive shafts in their own bearings in the slides, and that too can lead to a non-negligible amount of error in interpreting these twisting stresses in terms of the bending resistance of the bending zone.